<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    div{
        width:100px;
        height:50px;
        border:2px solid red;
    }

    .a{
        transition:width 2s linear 1s; /*1秒过后 div在2秒内匀速缓慢的变宽*/
    }

    div:hover{
        width:300px; /*触发 div变宽*/
    }
</style>
<body>
    
    <div class="a">Hello,拉勾</div>
</body>
</html>